In motive and standby applications, battery modules or cells are connected in series to provide desired voltage levels for various applications. Most of these battery systems in use today are operating without individual battery cell management. However, placing battery cells in series presents a problem with respect to the reliable and efficient recharging of the cells, specifically the overcharging or undercharging of the individual cells. Overcharged cells lose service life while undercharged ones fail to have their prescribed energy storage capacity.
Different battery cells loose their charges at different rates. Also, different cells have uneven leakage current, thereby yielding different shelf lives. For optimal battery performance it is best to charge all cells to the same voltage, while not overcharging any cell within the string. The importance of preventing overcharging varies with different types of batteries. For example, nicad batteries can be overcharged somewhat without causing damage to the battery. Thus, battery charging systems for nicad batteries have been devised which simply charge the battery without monitoring its charge and rapidly removing the battery from the charging system upon its becoming fully charged. However, other types of batteries, such as lithium ion batteries, are very sensitive to overcharging and become damaged thereby. Hence, these types of batteries cannot be used with nicad battery chargers without causing damage.
Unequalized charge state problems with individual battery cells in a string have been addressed previously. For example, individual battery cells have been monitored and charged first with a main charger and then individually charged using a relatively smaller independent charger, an example of such is that shown in U.S. Pat. No. 5,438,250. This method of charging batteries is inefficient and incapable of charging a string of batteries with a single power source.
Another method of battery management has been to use zener diodes to shunt current about fully charged individual cells, as shown in U.S. Pat. No. 4,719,401. Zener diodes however do not have a user selectable voltage level. Additionally, zener diodes have a leakage current of approximately 0.5 mA. A leakage current of this magnitude is often unacceptable as it significantly reduces the shelf life of the battery. Also, zener diode circuits cannot be easily adapted to charge different voltage batteries. As this restriction limits the zener diode circuit to one shunting voltage, the charger can only recharge batteries of one selected maximum voltage. However, it may be desired to have a charging circuit which may accept and recharge batteries of different potential capacities.
Battery recharging apparatuses have been designed wherein a battery shunting circuit is controlled through a series of diodes or a transistor, as shown in U.S. Pat. No. 5,387,857. With a series of diodes, the diodes are connected in series across the battery cell. Voltage is set to the maximum level of the fully charged battery cell. This arrangement however has a leakage current in the order of milli-amps before the diodes are turned on. This large leakage current significantly reduces the shelf-life of the battery. Also, the voltage across each forward biased diode is approximately 0.6 volts to 1.2 volts depending upon the current across it. This wide variation in voltages make it difficult to use several diodes to set a precise voltage level to protect a specific battery cell. Additionally, if the maximum voltage of a cell is large, it is impractical to use many diodes in series to bypass merely one cell.
With the transistor embodiment of U.S. Pat. No. 5,387,857, resistors are connected as a voltage divider across the battery cell. The divider sets the turn-on state of a transistor, which is shunted across the battery cell. To make the transistor bypass a large current, on the order of hundreds of milli-amps, as may be the case when the battery cell is fully charged, the resistors should be small enough to drive the transistor's base current to several milli-amps. These small resistors produce a very large leakage current across the battery cell. Thus, both these embodiments too are plagued with a large leakage current across the battery cell which significantly reduces the shelf-life of the battery. Also, when the transistor is turned on, the voltage across the collector and the emitter changes depending on the collector current. As such, it is difficult to set a precise voltage level. Additionally, the transistor has a slow turn-on speed which may lead to transient overcharging of a battery cell.
It thus is seen that a need remains for a rechargeable power supply that has efficient overcharge protection. Accordingly, it is to the provision of such that the present invention is primarily directed.